1. Field of the Invention
The invention deals with a control and power supply circuit for a signaling and/or lighting module, in particular for a motor vehicle. More particularly, the invention deals with such a control circuit for a signaling and/or lighting module handling at least two signaling and/or lighting functions from the same light sources. The invention also deals with a signaling and/or lighting module equipped with the circuit concerned. The invention also deals with the method of implementation by the control circuit and a microcontroller implementing the method.
2. Description of the Related Art
The use of one and the same group of light sources, in particular of light-emitting diode (LED) type, to handle at least two lighting functions is known in particular from the U.S. Patent Publication No. 2010/0066249 A1. In this teaching, a group of nine LEDs arranged according to a 3 by 3 matrix is powered selectively by two distinct power supplies, one corresponding to the “brake” function and the other corresponding to the “side light” (rear signaling light) function. The power supply corresponding to the “brake” function provides a lighting level greater than that of the “side light” function. In fact, the power supply for the “side light” function supplies a lower current because of the presence of an additional resistor.
The LEDs arranged in a matrix form three parallel branches, but with connections between the branches at each level. This type of connection means that, if one LED fails, the remaining LEDs of the corresponding branch will still be powered and will participate in the signaling. However, given that the current passing through these remaining LEDs will come from the LEDs of the higher level or levels of the other branches, the result will be a non-uniformity in the lighting power between the LEDs. Such a non-uniformity is not desirable for various reasons, in particular aesthetic and failure detection reasons. As it happens, the failure detection is particularly difficult because only the intensity of the current passing through all the LEDs can be monitored and the consumption difference should a single LED fail out of a group of nine means that there is a particular demand when it comes to measurement accuracy.
Furthermore, the brightness of the LEDs requires, in order to be monitored, a current whose intensity remains constant regardless of the input voltage. Their power supply is therefore significantly more complex than the power supplies for incandescent lamps, which simply have to be connected to a battery. In practice, the relationship which links the power supply voltage and current of the emissive semiconductors (the LEDs) is not linear. Thus, a small increase in voltage applied to the LED may lead to a significant increase in the current and therefore in the emitted light flux. The LEDs display a characteristic curve in direct voltage mode similar to that of the diodes. Below their switching threshold, or approximately 3.5 V for a white LED, very little current passes through them. On the other hand, above this threshold, as the direct voltage increases, the flow of the current increases exponentially. In the case of an LED power supply through an external resistor in series with the voltage source, if the power supply voltage increases by a little, the direct current passing through the LED is modified very substantially and the emission performance of the LED which is approximately proportional to the current which passes through it evolves accordingly. The series resistor must therefore be adjusted for each LED accordingly. This adjustment is valid only for a given temperature range and it is essential to take account of the thermal environment for the definition of the series resistor, or even have the latter evolved in temperature for the case of an extended temperature range. Furthermore, too great an increase in the power supply voltage for the LEDs through a resistor will result in an excessive current flow that may damage the component.
Furthermore, the chromatic coordinates of the white LEDs vary according to the power supply current passing through them. Thus, if a number of LEDs are driven in parallel (i.e., driven in voltage mode), the current which passes through each LED is different because of the intrinsic disparity between components associated with the technology and there may be significant differences in chromatic coordinates between each LED which are shown by the emitted whites having different colored aspects.
Backfitting a signaling device such as that of the teaching described above may pose problems in that one and the same electric circuit has to allow for two functions in order to satisfy the regulatory conditions. Indeed, the power ratio between the “brake” function and the “side light” function is fairly high, generally greater than a factor of 10. Furthermore, the regulation demands not only minimum lighting values but also maximum values.